Conventionally base plating for an insulating portion of a printed wiring board etc. is carried out based mainly on an electroless copper plating process. Meanwhile, many processes using the direct plating method, in which electroplating is carried out without performing the electroless copper plating, also exist in recent years. Examples of the general electroless plating process for plating the insulating portion include cleaning treatment→etching treatment→catalyst application treatment→electroless plating treatment. Furthermore, examples of the process using the direct plating method include cleaning treatment→etching treatment→catalyst application treatment→electrical-conductor layer forming treatment→electroplating treatment.
The catalyst application treatment is treatment of forming catalyst nuclei (Pd, Au, Ag, Pt, etc.) necessary for deposition of electroless plating on an insulating portion surface. For example, a method of forming palladium metal nuclei on an insulating portion surface by using a Pd—Sn colloidal solution or an alkaline palladium ion solution is known (Patent Document 1: U.S. Pat. No. 3,011,920).
If the Pd—Sn colloidal solution is used for the catalyst application treatment, treatment of removing Sn as a protective film (accelerator) is necessary after the catalyst application. If the accelerator is omitted, possibly the palladium catalytic activity is lowered and the plating reactivity decreases. Furthermore, possibly the connection reliability between the inner-layer copper and laminated copper, and the plating film is lowered.
A saturated halogen is necessary to stably keep the Pd—Sn colloid in the catalyst application solution, and generally the halogen concentration is adjusted by NaCl. However, a crystal (generally NaCl crystal) is often generated in a plating apparatus because of long-term use and corrosion of metal parts and troubles in operation of the apparatus often occur.
If the Pd—Sn colloidal solution is used for the catalyst application treatment, the colloidal metal is kept by divalent Sn (colloid protective film). When this divalent Sn is oxidized to quadrivalent Sn due to liquid circulation, possibly the characteristics of the colloid protective film are lost. Therefore, there is a problem that it is difficult to apply the Pd—Sn colloidal solution to an apparatus that requires strong liquid circulation like horizontal conveying apparatus. Furthermore, divalent Sn is oxidized to quadrivalent Sn due to entrained water in water rinse of pre-treatment and possibly the characteristics of the colloid protective film are lost. Therefore, pre-dip treatment needs to be performed between the water rinse and the Pd—Sn colloidal solution treatment to replace water on the surface of the object to be plated by a halide ion solution to thereby prevent the entrained water.
If the object to be plated is a substrate composed of an insulating portion and a copper portion like a printed wiring board, haloing due to dissolution of laminated copper inside a through-hole occurs and the substrate reliability is lowered in some cases. The haloing refers to the following phenomenon. An oxide of blackening treatment used for adhesion of a multilayer board is dissolved from an end of a hole due to permeation of the acid from the wall of the through-hole, so that a white or pink-like ring is generated at the periphery of the hole. If the haloing occurs, particularly in the case of a circuit in which through-holes are formed at high density, electrical contact with the adjacent through-hole on the circuit occurs. Furthermore, the adhesion between resins deteriorates, so that permeation of the catalyst application solution into the laminated portion and lamination separation (delamination) occur. The blackening treatment is to form a copper oxide film on the inner-layer copper surface and give minute recesses and projections in order to enhance the adhesion by lamination press of the inner-layer copper and the resin. By this treatment, the adhesion is enhanced based on the anchor effect.
Furthermore, displacement deposition of palladium on copper occurs due to dissolution of the copper on the substrate and the deposited palladium adversely affects the connection reliability between the laminated copper and the plating film in some cases. Moreover, the copper on the substrate dissolves into the catalyst application solution and thus renewal of the catalyst application solution is necessary, which leads to a problem of cost increase.
To solve these problems, a catalyst application solution composed of a strongly-acidic palladium colloidal solution that does not use Sn and contains an inorganic acid as solvent has been proposed (Patent Document 2: JP-A S61-166977). This palladium colloidal solution is strongly acidic although not using Sn. If the strongly-acidic palladium colloidal solution is used as the catalyst application solution for plating treatment for a printed wiring board, there is a problem that the acid in the solution dissolves the laminated copper of the printed wiring board. Furthermore, there is a problem that the dissolved copper (Cu2+) is reduced by a reducer in the catalyst application solution to form a copper (Cu0) colloid or adhere to the palladium colloid and exist as a colloid and therefore the activity as the catalyst in the electroless copper plating treatment is lowered.
On the other hand, if a conventional strongly-alkaline palladium ion solution is used as the catalyst application solution, reduction treatment (reducer) to reduce the palladium ion complex to the palladium metal is necessary (Patent Document 3: JP-A H8-316612). This is because the palladium ion complex itself does not act as the catalyst of electroless (copper) plating.
It is difficult to use the alkaline palladium ion solution for a base material that does not have alkali resistance (e.g. polyimide layer or adhesive layer portion) because the solution eats away the base material to cause abnormal plating and non-plating. Furthermore, the amount of palladium adsorption to the base material is about half compared with the case of using the Pd—Sn colloidal solution or the strongly-acidic palladium colloidal solution. In the case of a smooth base material having a small surface area, a non-plating problem occurs because necessary amount of palladium is insufficient due to instantaneous reaction of electroless copper plating.
Prior-art documents relating to the present invention include, besides the above-described documents, JP-A 2007-16283 (Patent Document 4).